The present invention relates to process devices of the type used in industrial processes. More particularly, the present invention relates to diagnostics of such process devices.
Field devices such as process controllers, monitors and transmitters, are used in the process control industry to remotely control or sense a process variable. For example, a process variable may be transmitted to a control room by a transmitter for use in controlling the process or for providing information about process operation to a controller. For example, information related to pressure of process fluid may be transmitted to a control room and used to control the process, such as oil refining.
One typical prior art technique for transmitting information involves controlling the amount of power flowing through a process control loop. Current is supplied from a current source in the control room and the transmitter controls the current from its location in the field. For example, a 4 mA signal can be used to indicate a zero reading and a 20 mA signal can be used to indicate a full scale reading. More recently, transmitters have employed digital circuitry which communicates with the control room using a digital signal which is superimposed onto the analog current signal flowing through the process control loop. One example of such a technique is the HART® communication protocol proposed by Rosemount Inc. The HART® protocol and other such protocols typically include a set of commands or instructions which can be sent to the transmitter to elicit a desired response, such as transmitter control or interrogation.
Fieldbus is a communications protocol proposed by the Fieldbus Foundation and is directed to defining a communications layer or protocol for transmitting information on a process control loop. In the Fieldbus protocol, the current flowing through the loop is not used to transmit an analog signal. Instead, all information is digitally transmitted. Further, the Fieldbus standard, and a standard known as Profibus, allow transmitters to be configured in a multi-drop configuration in which more than one transmitter is connected on the same process control loop. Other communication protocols include the MODBUS® protocol and Ethernet. In some configurations, two, three, four or any number of wires can be used to connect to the process device, including non-physical connections such as RF (radio frequency).
It is often desirable to monitor operation of process devices. One device which provides built-in test equipment is shown in U.S. Pat. No. 5,481,200 entitled FIELD TRANSMITTER BUILT-IN TEST EQUIPMENT.
When a process device fails, it is often necessary to shut down the entire process so that the failed device can be repaired or replaced. Typically, it is not possible to predict an impending failure of a process device prior to its occurrence. Thus, when the process device does fail, it occurs unexpectedly, and may require the unexpected shut down of the entire process. Although various attempts have been made at predicting an impending failure prior to its occurrence, there is an ongoing need for such a technique. Prior prediction of a pending failure allows the failing device to be replaced as desired prior to its ultimate failure.